Guidance Computer

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Description

The Guidance Computer is a specialized Computer that manages the attitude determination and control software for a spacecraft. It constructs and maintains a software chain sequence consisting of navigation, pointing, controller, and mapping modules. The computer enables runtime switching between different guidance modes without manual software reconfiguration, automatically handling message routing between active chains.

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Example Use Cases

• Mode-Based Operations: Switch between sun-pointing for power generation and nadir-pointing for Earth observation during different mission phases.
• Simplified GNC Integration: Add a guidance computer to a spacecraft and select chain types without manually constructing the full software stack.
• Autonomous Guidance: Implement ground-commanded or autonomous mode transitions based on spacecraft state.

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Module Implementation

Software Chain Architecture

The guidance computer creates a GuidanceSoftwareChainSequence containing four chain categories that execute sequentially to produce actuator commands from sensor data. Each category can contain multiple chain implementations, but only one chain per category is active at any time.

Chain Category	Description	Output Messages
Navigation	Estimates spacecraft position and attitude from sensor data	NavigationTranslationMessage, NavigationAttitudeMessage
Pointing	Computes the reference attitude for target alignment	AttitudeReferenceMessage, AttitudeErrorMessage
Controller	Computes control torque to achieve the desired attitude	CommandTorqueMessage
Mapping	Maps the command torque to actuator-specific commands	Actuator-specific messages

Available Chains

Chain	Category	Purpose
Simple	Navigation	Applies configurable errors to the true spacecraft state
Sun Estimation	Navigation	Estimates attitude using sun sensor measurements
Inertial	Pointing	Maintains alignment with an inertial reference frame
Sun	Pointing	Aligns a body axis toward the sun
Velocity	Pointing	Aligns a body axis along the orbital velocity vector
Nadir	Pointing	Aligns a body axis toward a celestial body center
Ground	Pointing	Aligns a body axis toward a ground location
Relative	Pointing	Aligns a body axis toward another spacecraft
Dock	Pointing	Aligns the spacecraft for docking approach
Idle	Controller	Produces zero torque commands
MRP	Controller	Modified Rodrigues Parameter feedback controller
Reaction Wheels	Mapping	Distributes torque commands across a reaction wheel array
Magnetic Torque Bars	Mapping	Maps torque commands to magnetic torquer dipoles
Thrusters	Mapping	Maps torque commands to thruster firings

Mode Switching

Each chain category exposes a mode property that can be modified at runtime:

• NavigationMode
• PointingMode
• ControllerMode
• MappingMode

When a mode property changes, the guidance computer disables the previous chain and enables the new chain. Message routing between adjacent chain categories is automatically updated so that output messages from the newly activated chain connect to the appropriate downstream consumers.

Message Routing

The guidance computer maintains consolidated output messages for each stage of the guidance pipeline. These messages are updated each simulation step by copying data from the currently active chain:

Output Message	Source Chain	Contents
Out_NavigationTranslationMsg	Navigation	Position ${\mathbf{r}}_{B/N}$, velocity ${\mathbf{v}}_{B/N}$, velocity accumulation
Out_NavigationAttitudeMsg	Navigation	Attitude ${\mathbf{\sigma }}_{B/N}$, angular rate ${\mathbf{\omega }}_{B/N}$, sun direction
Out_AttitudeReferenceMsg	Pointing	Reference attitude ${\mathbf{\sigma }}_{R/N}$, reference rate ${\mathbf{\omega }}_{R/N}$, rate derivative
Out_AttitudeErrorMsg	Pointing	Attitude error ${\mathbf{\sigma }}_{B/R}$, rate error ${\mathbf{\omega }}_{B/R}$
Out_CommandTorqueMsg	Controller	Commanded torque ${\mathbf{L}}_r$ in body frame

This architecture allows external modules to subscribe to the consolidated output messages without needing to track which specific chain is currently active.

Computer State Integration

The guidance software sequence lifecycle is tied to the computer state:

• Running State: The sequence is enabled when the computer transitions to running, allowing all active chains to execute.
• Shutdown State: The sequence is disabled when the computer shuts down, halting chain execution.

The LoadSoftware method initializes all chains in reverse order (mapping, controller, pointing, navigation) to ensure message connections are established correctly before execution begins.

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Assumptions/Limitations

• Only one chain per category can be active at any time.
• All chains within a category must produce compatible output messages for downstream consumers.
• Chain-specific parameters are configured through dedicated accessor methods on the guidance computer.
• The computer must reach the running state before software chains will execute.
• Mode changes take effect on the next simulation update, not instantaneously.